Apparatus for switching nominal and attenuated power between ablation probes

ABSTRACT

A tissue ablation system comprises an ablation source, such as an RF ablation source, configured for generating a common power signal, and a power multiplexor configured for splitting the power signal into first and second power signals, substantially attenuating the second power signal relative to the first power signal to create nominal and attenuated power signals, and sequentially delivering the nominal power signal to each tissue ablation probe, while delivering the attenuated power signal to the remaining ablation probes to which the nominal power signal is currently not delivered.

FIELD OF THE INVENTION

The invention relates generally to the structure and use ofradiofrequency electrosurgical apparatus for the treatment of tissue.More particularly, the invention relates to an electrosurgical systemhaving multiple ablation probes to treat large volumes of tissue,particularly for the treatment of tumors in the liver and other tissuesand organs.

BACKGROUND

The delivery of radio frequency (RF) energy to target regions withintissue is known for a variety of purposes of particular interest to thepresent invention(s). In one particular application, RF energy may bedelivered to diseased regions (e.g., tumors) for the purpose of ablatingpredictable volumes of tissue with minimal patient trauma. RF ablationof tumors is currently performed using one of two core technologies.

The first technology uses a single needle electrode, which when attachedto a RF generator, emits RF energy from the exposed, non-insulatedportion of the electrode. This energy translates into ion agitation,which is converted into heat and induces cellular death via coagulationnecrosis. The second technology utilizes multiple needle electrodes,which have been designed for the treatment and necrosis of tumors in theliver and other solid tissues. PCT application WO 96/29946 and U.S. Pat.No. 6,379,353 disclose such probes. In U.S. Pat. No. 6,379,353, a probesystem comprises a cannula having a needle electrode arrayreciprocatably mounted therein. The individual electrodes within thearray have spring memory, so that they assume a radially outward,arcuate configuration as they are advanced distally from the cannula. Ingeneral, a multiple electrode array creates a larger lesion than thatcreated by a single needle electrode.

In theory, RF ablation can be used to sculpt precisely the volume ofnecrosis to match the extent of the tumor. By varying the power outputand the type of electrical waveform, it is possible to control theextent of heating, and thus, the resulting ablation. However, the sizeof tissue coagulation created from a single electrode, and to a lesserextent a multiple electrode array, has been limited by heat dispersion.As a consequence, when ablating lesions that are larger than thecapability of the above-mentioned devices, the common practice is tostack ablations (i.e., perform multiple ablations) within a given area.This requires multiple electrode placements and ablations facilitated bythe use of ultrasound imaging to visualize the electrode in relation tothe target tissue. Because of the echogenic cloud created by the ablatedtissue, however, this process often becomes difficult to accuratelyperform. This process considerably increases treatment duration andpatent discomfort and requires significant skill for meticulousprecision of probe placement.

In response to this, the marketplace has attempted to create largerlesions with a single probe insertion. Increasing generator output,however, has been generally unsuccessful for increasing lesion diameter,because an increased wattage is associated with a local increase oftemperature to more than 100° C., which induces tissue vaporization andcharring. This then increases local tissue impedance, limiting RFdeposition, and therefore heat diffusion and associated coagulationnecrosis. In addition, patient tolerance appears to be at the maximumusing currently available 200 W generators.

To a large extent, the size and nature of an ablation lesion depends onhow the electrode element(s) are arranged. In one arrangement, RFcurrent may be delivered to an electrode element (whether a singleelectrode or electrode array) in a monopolar fashion, which means thatcurrent will pass from the electrode element to a dispersive electrodeattached externally to the patient, e.g., using a contact pad placed onthe patient's flank. In another arrangement, the RF current is deliveredto two electrode elements in a bipolar fashion, which means that currentwill pass between “positive” and “negative” electrode elements. Bipolararrangements, which require the RF energy to traverse through arelatively small amount of tissue between the tightly spaced electrodes,are more efficient than monopolar arrangements, which require the RFenergy to traverse through the thickness of the patient's body. As aresult, bipolar electrode arrays generally create larger and/or moreefficient lesions than monopolar electrode arrays. To provide evenlarger lesions, it is known to operate two electrode arrays in a bipolararrangement.

Thus, to a certain extent, the use of bipolar electrode arrangements haseliminated the need to “stack” ablations when treating a tumor. Theability to provide uniform heating and the creation of homogenous tissuelesions, however, is particularly difficult with bipolar devices. Forexample, the two bipolar electrodes may be placed in regions with quitedifferent perfusion characteristics, and the heating around each polecan be quite different. That is, one pole may be located adjacent to alarge blood vessel, while the other pole may be located adjacent totissue, which is less perfused. Thus, the pole located in the lessperfused tissue will heat the tissue immediately surrounding theelectrode much more rapidly than the tissue surrounding the oppositepolar electrode is heated. In such circumstances, the tissue surroundingone pole may be preferentially heated and necrosed, while the tissuesurrounding the other pole will neither be heated nor necrosedsufficiently.

In the case where two electrode arrays are used, if the distance betweenthe electrode arrays is too great in an attempt to ablate a longertissue volume, the energy transmitted between the electrode arrays maythin and not fully ablate the intermediate tissue. As a result, anhour-glass shaped ablation, rather than the desired uniformcircular/elliptical ablation, may be created. Also, because theelectrode arrays are, in effect, three-dimensional, portions between theelectrode arrays will be closer together than other portions of theelectrode arrays, thereby causing a non-uniform current density betweenthe electrode arrays, resulting in a non-uniform ablation. Besideslacking the ability to produce predictable homogenous lesions, bipolararrangements, which are designed to ablate tissue between theelectrodes, are not well-suited for simultaneously ablating multipletissue regions.

In situations where it is desired to produce large homogenous lesions orsimultaneously ablate multiple tissue regions, it is known to arrangemultiple probes in a monopolar fashion (i.e., the RF energy generated byeach probe is conveyed to a dispersive electrode attached to the skin ofthe patient. In this case, current flows from each probe to the groundpad. A drawback to this approach is that simultaneously supplying powerto multiple probes taxes the power output by the RF generator, which maycause insufficient heating around the probes. Also, because the tissueadjacent the probes is non-uniform (e.g., one probe may be adjacent ablood vessel), the heating pattern created by the probes will benon-uniform, thereby making it difficult to predict the nature of theresulting lesion.

To address these drawbacks, it is known to use an ablation system thatsequentially switches ablation energy between probes, so that at anygiven time, ablation energy is supplied to only one probe. While thisswitching technique may result in a more efficient and predictablelesion, it is believed that, during any given time period, the tissueadjacent the probes to which the ablation energy is not currentlysupplied temporarily cools—especially when the switching speed betweenthe probes is relatively slow, e.g., a few seconds. As a result, thecooled tissue must be reheated when power is again supplied to theadjacent probes, thereby losing some efficiency in the ablation process.

For this reason, it would be desirable to provide improved multi-probeelectrosurgical methods and systems for more efficiently ablating tumorsin the liver and other body organs.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, a tissueablation system is provided. The tissue ablation system comprises anablation source, such as a radio frequency (RF) source, having a commonpower output. The tissue ablation system further comprises a powersplitter having a splitter input and first and second splitter outputs.The splitter input is coupled to the common power output. In oneembodiment, the first and second splitter outputs are substantiallyunattenuated relative to the common power output, although it should benoted that the present inventions should not be so limited. For example,an intervening attenuation device, whatever its nature, can be placedbetween the common power output and the first and second splitterinputs, so that the first and second splitter outputs are bothattenuated relative to the common power output.

The tissue ablation system further comprises a power attenuator havingan attenuator input and an attenuator output. The attenuator input iscoupled to the second splitter output. In one embodiment, the attenuatoris configured for attenuating power at a level equal to or greater than3 dB, e.g., within the range of 3 dB to 6 dB. Although the attenuatormay be fixed in one embodiment, the attenuator may be variable inalternative embodiments. For example, the attenuation value of theattenuator may be capable of being set by the user. As another example,the tissue ablation system may further comprise a feedback controlcircuit configured for receiving a feedback input (e.g., a measuredphysiological parameter, such as tissue impedance or temperature) andfor varying the attenuation value of the attenuator based on thefeedback input.

The tissue ablation system further comprises a switch having first andsecond switch inputs and a plurality of switch outputs. The first switchinput is coupled to the first splitter output and the second switchinput is coupled to the attenuator output. The plurality of switchoutputs are coupleable or coupled to a plurality of tissue ablationprobes, which may be included within the tissue ablation system. In oneembodiment, the plurality of switch outputs comprises at least threeoutputs to accommodate at least three ablation probes, although otherplural numbers of switch outputs can be used including just two.

The switch is configured for sequentially coupling the first switchinput (which is coupled to the first splitter output) to each one of theswitch outputs, while coupling the second switch input (which is coupledto the attenuator output) to the switch output currently not coupled tothe first switch input. Although the present inventions should not be solimited in their broadest aspects, this configuration allows the powerto be continuously delivered to each ablation probe—albeit sometimes atan attenuated level.

The switch may be configured for sequentially coupling the first switchinput to each switch output at a fixed rate. Preferably, this fixedswitching rate is greater than once per second to maximize the ablationefficiency of the system, but less than thirty seconds to prevent tissuecharring. It should be noted, however, that the present inventions intheir broadest aspects should not be so limited. As such, the fixedswitching rate may be less than once per one second or greater than onceper thirty seconds if desirable. The switch may alternatively beconfigured for sequentially coupling the first switch input to eachswitch output at a variable rate. For example, the tissue ablationsystem may further comprise a feedback control circuit configured forreceiving a feedback input (e.g., a measured physiological parameter,such as tissue impedance or temperature) and controlling the switch tocouple the first splitter output to each switch output based on thefeedback input.

Notably, for the purposes of this specification, two elements that arecoupled or coupleable together does not necessarily mean that the twoelements must be connected together. Rather, the one element need onlybe capable of receiving power or derivation of that power from thatother element. Thus, the two elements may be coupled or coupleabletogether even though an intervening element exists between the twoelements.

In accordance with a second aspect of the present inventions, anothertissue ablation system is provided. The tissue ablation system comprisesan ablation source, such as an RF ablation source, configured forgenerating a common power signal. The tissue ablation system furthercomprises a power multiplexor configured for splitting the power signalinto first and second power signals, and substantially attenuating thesecond power signal relative to the first power signal. In oneembodiment, the first power signal is substantially unattenuatedrelative to the common power signal, although it should be noted thatthe present inventions should not be so limited. For example, anintervening attenuation device can be used to attenuate the first andsecond power signals after they are output by the ablation source.

In one embodiment, the power multiplexor is configured for attenuatingpower at a level equal to or greater than 3 dB, e.g., within the rangeof 3 dB to 6 dB. In an optional embodiment, the power multiplexor isconfigured for varying the value that the second power signal isattenuated. In this case, the power multiplexor may optionally beconfigured for receiving a feedback input (e.g., a measuredphysiological parameter, such as tissue impedance or temperature) andvarying the attenuation value based on the feedback input.

The power multiplexor is further configured for delivering the firstpower signal to one or more of a plurality of tissue ablation probes(which may be included within the tissue ablation system), whiledelivering the second power signal to a different one or more of theplurality of ablation probes. The power multiplexor may be configuredfor varying the number of tissue ablation probes to which it deliversthe first and second power signals. In this manner, the tissue ablationsystem can be adapted to any number of tissue ablation probes desired tobe used.

By way of non-limiting example, the power multiplexor may be configuredfor sequentially delivering the first power signal to different sets ofthe tissue ablation probes, while delivering the second power signal tothe tissue ablation probes to which the first power signal is notcurrently delivered. The set of the tissue ablation probes to which thefirst power signal is sequentially delivered can equal any value, but inthe preferred embodiment, the probe set only comprises a single tissueablation probe in order to focus more of the ablation energy towards asingle tissue ablation probe at a time. The plurality of tissue ablationprobes may equal any plurality number, including just two ablationprobes, but in one embodiment, comprises at least three tissue ablationprobes.

If the first power signal is sequentially delivered, the powermultiplexor may be configured for sequentially deliver it to each tissueablation probe at a fixed rate. Preferably, this fixed rate is greaterthan one probe per second to maximize the ablation efficiency of thesystem, but less than one probe per thirty seconds to prevent tissuecharring. It should be noted, however, that the present inventions intheir broadest aspects should not be so limited. As such, the fixed ratemay be less than one probe per one second or greater than one probe perthirty seconds if desirable. The power multiplexor may alternatively beconfigured for sequentially coupling the first power signal to eachtissue ablation probe at a variable rate. For example, the powermultiplexor may be configured for receiving a feedback input (e.g., ameasured physiological parameter, such as tissue impedance ortemperature) and delivering the first power signal to each tissueablation probe based on the feedback input.

In accordance with a third aspect of the present inventions, a method oftreating tissue within a patient is provided. The method comprisesintroducing a plurality of probes into the patient. In one method, theprobes are percutaneously introduced into the patient, although theprobes may alternatively be intravascularly introduced into the patientor introduced through an open surgical incision. In one method, at leastthree ablation probes are used, although other plural numbers of probescan be used, including just two.

The method further comprises sequentially delivering nominal ablationenergy (e.g., RF ablation energy) to each of a plurality of probes,while delivering attenuated ablation energy to a remainder of theplurality of probes. The nominal ablation energy may or may not beattenuated. However, the attenuated ablation energy has an amplitudethat is substantially less than the amplitude of the nominal ablationenergy. For example, the amplitude of the attenuated ablation energy maybe equal to or greater than 3 dB below the amplitude of the nominalablation energy, e.g., within the range of 3 to 6 dB.

The nominal ablation energy may be sequentially delivered to each probeat a fixed rate. Preferably, this fixed rate is greater than one probeper second to maximize the ablation efficiency of the system, but lessthan thirty seconds to prevent tissue charring. It should be noted,however, that the present inventions in their broadest aspects shouldnot be so limited. As such, the fixed rate may be less than one probeper one second or greater than one probe per thirty seconds ifdesirable. The nominal ablation energy may alternatively be sequentiallydelivered to each probe at a variable rate. For example, a physiologicalparameter, e.g., a tissue impedance or temperature, can be measured, inwhich case, the nominal ablation energy can be sequentially delivered tothe probes based on the measured physiological parameters.

In one method, the nominal ablation energy and attenuated ablationenergy are derived from a single ablation source. In alternativemethods, the nominal ablation energy and attenuated ablation energy canbe derived from multiple ablation sources. The amplitude of theattenuated ablation energy may be variably set, e.g., by measuring aphysiological feedback parameter and setting the amplitude basedthereon, or even by manually setting the amplitude.

The method further comprises ablating the tissue with the nominalablation energy and attenuated ablation energy. The tissue may be anytissue that can be treated with ablation energy, e.g., one or moretumors. In one method, the tissue is distributed amongst a plurality oftreatment regions, but may also be contained within a single treatmentregion.

In accordance with a fourth aspect of the present inventions, anothermethod of treating tissue within a patient is provided. The methodcomprises introducing a plurality of probes into the patient, which maybe accomplished in the same manner described above. The method furthercomprises delivering nominal ablation energy (e.g., RF ablation energy)to one or more of the ablation probes, while delivering attenuatedablation energy to a different one or more of the ablation probes, andablating the tissue with the nominal ablation energy and attenuatedablation energy. The detailed features of this method may be the similarto those described above, with the exception that, in one embodiment,the first power signal may be sequentially delivered to more than onetissue ablation probe at a time.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF DRAWINGS

The drawings illustrate the design and utility of preferredembodiment(s) of the invention, in which similar elements are referredto by common reference numerals. In order to better appreciate theadvantages and objects of the invention, reference should be made to theaccompanying drawings that illustrate the preferred embodiment(s). Thedrawings, however, depict the embodiment(s) of the invention, and shouldnot be taken as limiting its scope. With this caveat, the embodiment(s)of the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a perspective view of a tissue ablation system constructed inaccordance with a preferred embodiment of the present invention;

FIG. 2 is a partially cutaway side view of an ablation probe used in thetissue ablation system of FIG. 1, wherein an array of electrode tines isparticularly shown retracted;

FIG. 3 is a partially cutaway side view of an ablation probe used in thetissue ablation system of FIG. 1, wherein an array of electrode tines isparticularly shown deployed;

FIG. 4 is a timing diagram illustrating the switching of ablation energybetween tissue ablation probes used in the tissue ablation system ofFIG. 1;

FIG. 5 is a table illustrating the ablation states in which theplurality of tissue ablation probes used in the tissue ablation systemof FIG. 1 can be placed;

FIG. 6 is a detailed schematic diagram of a power multiplexor used inthe tissue ablation system of FIG. 1;

FIG. 7 is a perspective view of a tissue ablation system constructed inaccordance with another preferred embodiment of the present invention;

FIG. 8 is a timing diagram illustrating the switching of ablation energybetween tissue ablation probes used in the tissue ablation system ofFIG. 7;

FIG. 9 is a table illustrating the ablation states in which theplurality of tissue ablation probes used in the tissue ablation systemof FIG. 7 can be placed;

FIG. 10 is a detailed schematic diagram of a power multiplexor used inthe tissue ablation system of FIG. 7; and

FIGS. 11A-11F are side views illustrating a method of ablating tissueusing the tissue ablation system of FIG. 1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Referring generally to FIG. 1, a tissue ablation system 10 constructedin accordance with one embodiment of the present inventions will bedescribed. The tissue ablation system 10 generally includes a pluralityof tissue ablation probes 12 (in this case, three probes 12(1)-(3)) forintroduction into the body of a patient for ablative treatment of targettissue, a radio frequency (RF) generator 14 configured for generating RFpower, and a power multiplexor 16 configured for receiving the RF powerfrom the RF generator via a standard RF cable 15 and selectivelyproviding the RF power to the ablation probes 12 via standard RF cables13 in accordance with a particular pattern, such that each ablationprobe 12 is always “on” during a tissue ablation procedure—albeit at apower level that may be substantially below the power level of the RFgenerator 14.

Referring specifically now to FIGS. 2 and 3, each probe 12 includes anelongate cannula 18 having a proximal end 20, a distal end 22, and acentral lumen 24, a probe shaft 26 slidably disposed within the cannulalumen 24 and having a proximal end 28 and a distal end 30, and an arrayof electrode tines 32 carried by the distal end 28 of the probe shaft26. The cannula 18 may be rigid, semi-rigid, or flexible depending uponthe designed means for introducing the cannula 18 to the target tissue.The probe shaft 26 is composed of a suitably rigid material, such asplastic, metal or the like.

Each probe 12 further includes a handle assembly 34, which includes ahandle member 36 mounted to the proximal end 26 of the probe shaft 26,and a handle sleeve 38 mounted to the proximal end 20 of the cannula 18.The handle member 36 is slidably engaged with the handle sleeve 38 (andthe cannula 18). The handle member 36 and handle sleeve 38 can becomposed of any suitable rigid material, such as, e.g., metal, plastic,or the like. The handle assembly 34 also includes an electricalconnector 40 mounted within the handle member 36. The electricalconnector 40 is electrically coupled to the electrode array 32, e.g.,via the probe shaft 26 (which will be electrically conductive) orseparate wires (not shown). The electrical connector 40 is configuredfor mating with the proximal end of a RF cable 13 (shown in FIG. 1).Alternatively, the RF cable 13 may be hardwired within the handle member36.

It can be appreciated that longitudinal translation of the probe shaft26 relative to the cannula 18 in a distal direction 42, by holding thehandle sleeve 38 and displacing the handle member 36 in the distaldirection 42, deploys the electrode array 32 from the distal end 22 ofthe cannula 18 (FIG. 3), and longitudinal translation of the probe shaft26 relative to the cannula 18 in a proximal direction 44, by holding thehandle sleeve 38 and displacing the handle member 36 in the proximaldirection 44, retracts the probe shaft 26 and the electrode array 32into the distal end 22 of the cannula 18 (FIG. 2).

In the illustrated embodiment, the RF current is delivered to theelectrode array 32 in a monopolar fashion, which means that current willpass from the electrode array 32, which is configured to concentrate theenergy flux in order to have an injurious effect on the surroundingtissue, and a dispersive electrode (not shown), which is locatedremotely from the electrode array 32 and has a sufficiently large area(typically 130 cm² for an adult), so that the current density is low andnon-injurious to surrounding tissue. In the illustrated embodiment, thedispersive electrode may be attached externally to the patient, e.g.,using a contact pad placed on the patient's flank.

Further details regarding electrode array-type probe arrangements aredisclosed in U.S. Pat. No. 6,379,353, which is hereby expresslyincorporated by reference. It should be noted that the tissue ablationprobe 12 illustrated in FIGS. 2 and 3 is only one type of ablation probethat can be used with the tissue treatment system 10. For example, asingle needle electrode probe may be used as well.

Referring back to FIG. 1, the RF generator 14 may be a conventionalgeneral purpose electrosurgical power supply operating at a frequency inthe range from 300 kHz to 5 MHz, with a conventional sinusoidal ornon-sinusoidal wave form. Such power supplies are available from manycommercial suppliers, such as Valleylab, Aspen, Bovie, and Ellman. Mostgeneral purpose electrosurgical power supplies, however, are constantcurrent, variable voltage devices and operate at higher voltages andpowers than would normally be necessary or suitable. Thus, such powersupplies will usually be operated initially at the lower ends of theirvoltage and power capabilities, with voltage then being increased asnecessary to maintain current flow. More suitable power supplies will becapable of supplying an ablation current at a relatively low fixedvoltage, typically below 200 V (peak-to-peak). Such low voltageoperation permits use of a power supply that will significantly andpassively reduce output in response to impedance changes in the targettissue. The output will usually be from 50 W to 300 W, usually having asinusoidal wave form, but other wave forms would also be acceptable.Power supplies capable of operating within these ranges are availablefrom commercial vendors, such as Boston Scientific TherapeuticsCorporation. Preferred power supplies are model RF-2000 and RF-3000,available from Boston Scientific Corporation.

Referring still to FIG. 1, the power multiplexor 16 comprises a commoninput connector 17 coupled to an RF output connector 19 of the RFgenerator 14 via the RF cable 15, and a plurality of output connectors21 (in this case, three output connectors 21(1)-(3)) coupled to theelectrical connectors 40 of the ablation probes 12 via the RF cables 13.Although the power multiplexor 16 is shown external to the RF generator14, the power multiplexor 16 can alternatively be incorporated into RFgenerator 14 itself. However, it is believed that an external powermultiplexor 16 will allow the tissue ablation system 10 to be moreeasily implemented with standard RF generators simply by connecting thepower multiplexor 16 between the tissue ablation probes 12 and RFgenerator 14, as well as to provide physicians the flexibility ofoptionally using the RF generator 14 in a standard manner by directlyconnecting a single tissue ablation probe 12 to the RF generator 14without the use of the power multiplexor 16.

In any event, the power multiplexor 16 is configured for splitting thepower signal from the output connector 19 of the RF generator 14 intotwo power signals, substantially attenuating one of the power signalsrelative to the other power signal, and presenting the power signals tothe output connectors 21, and thus, the ablation probes 12 in accordancewith a particular pattern in a manner that provides at least some powerto all of the ablation probes 12 during any given time period.

Although it is preferred that one of the power signals remainssubstantially unattenuated, both power signals may be attenuated ifdesired as long as one of the power signals is substantially moreattenuated than the other power signal. For example, after attenuation,the second power signal may be 1-30 dB less than the first power signal,but preferably is 3-6 dB less than the first power signal. For purposesof brevity and clarity, the power signal that exhibits substantiallyless power than the other power signal will be considered the“attenuated signal” and the signal that exhibits substantially morepower than the other signal will be considered the “nominal signal.”

It should be noted the power signals will sometimes be described hereinas being presented to the output connectors 21, and at other times willbe described herein as being delivered to ablation probes 12. When apower signal is described as being presented to an output connector 21,it follows that such power signal will be delivered to an ablation probe12 connected to the output connector 21. In a similar fashion, when apower signal is described as being delivered to an ablation probe 12, itfollows that such power signal is presented to the output connector 21to which the ablation probe 12 is connected.

The particular pattern followed by the power multiplexor 16 indelivering the power signals to the ablation probes 12 involvessequentially delivering the nominal power signal to different sets ofthe ablation probes 12, while delivering the attenuated power signal toother sets of the ablation probes 12. In the illustrated embodiment,each set of ablation probes 12 to which the nominal power signal isdelivered contains a single ablation probe 12, and thus, each set ofablation probes 12 to which the attenuated power signal is deliveredcontains the remaining two ablation probes 12.

For example, as illustrated in FIG. 4, the power multiplexor 16 deliversthe nominal power signal to the first ablation probe 12(1) during afirst time period T1, while delivering the attenuated power signal tothe second and third ablation probes 12(2), (3). The power multiplexor16 then delivers the nominal power signal to the second ablation probe12(2) during a second time period T2, while delivering the attenuatingpower signal to the first and third ablation probes 12(1), (3). Thepower multiplexor 16 then delivers the nominal power signal to the thirdablation probe 12(3) during a third time period T3, while delivering theattenuating power signal to the first and second ablation probes 12(1),(2). This pattern is then repeated for subsequent time periods untilcompletion of the ablation procedure. Thus, it can be appreciated thatthe plurality of ablation probes 12 may be placed within three differentablation states, as illustrated in FIG. 5. The timing of the pattern maybe controlled in any one or more of a variety of manners, as will bedescribed in further detail below.

Although the set of ablation probes 12 to which the nominal power signalis delivered is described as containing a single ablation probe, and theset of ablation probes 12 to which the attenuated power signal isdelivered is described as containing the remaining two ablation probes,it should be noted that other probe set configurations are possible. Forexample, the nominal power signal may be sequentially delivered todifferent pairs of ablation probes 12, while the attenuated power signalis delivered to the remaining ablation probe 12. It should be noted,however, that it is preferred that the nominal power signal be deliveredto a single ablation probe at a time in order to maximize the efficiencyof the tissue ablation system 10.

The power multiplexor 16 may be implemented in any one of a variety ofmanners in order to effect the power signal attenuation and switchingfunctions described above. In the embodiment illustrated in FIG. 6, thepower multiplexor 16 comprises a power splitter 46 having a common inputterminal 48 coupled to the common input connector 17 of the powermultiplexor 16. The power splitter 46 is configured for splitting thepower from the RF generator 14, so that it is presented at first andsecond output terminals 50, 52.

The power multiplexor 16 further comprises a power attenuator 54 havingan input terminal 56 coupled to one of the first and second outputterminals 50, 52 of the power splitter 46, and an output terminal 58 forpresentation of the attenuated power signal. In the illustratedembodiment, the input terminal 56 of the power attenuator 54 is coupledto the second output terminal 52 of the power splitter 46—although theinput terminal 56 of the power attenuator 54 can alternatively becoupled to the first output terminal 50 of the power splitter 46instead. The value of the power attenuator 54 may fall within the rangeof 1-30 dB, preferably within the range of 3-6 dB. In the illustratedembodiment, the attenuator 54 has a fixed attenuation value, althoughthe attenuation value may alternatively be variable, as discussed infurther detail below. Thus, it can be appreciated that the nominal powersignal is presented at the first output terminal 50 of the powersplitter 46, while the attenuated power signal is presented at theoutput terminal 58 of the power attenuator 54.

The power multiplexor 16 further comprises a switch device 59 comprisinga plurality of switch elements 60 (in this case, three switch elements60(1)-(3)). Each switch element 60 takes the form of a single-poledouble-throw (SPDT) switch element that comprises a first input terminal62 coupled to the first output terminal 50 of the power splitter 46 forreceiving the nominal power signal, and a second input terminal 64coupled to the output terminal 58 of the attenuator 54 for receiving theattenuated power signal. Each switch element 60 further comprises anoutput terminal 66 coupled to the respective one of the previouslydescribed multiplexed output connectors 21 of the power multiplexor 16for presentation of either the nominal power signal or the attenuatedpower signal thereon. That is, the output terminal 66 of the firstswitch element 60(1) is coupled to the first output connector 21(1), theoutput terminal 66 of the second switch element 60(2) is coupled to thesecond output connector 21(2), and the output terminal 66 of the thirdswitch element 60(3) is coupled to the third output connector 21(3).Each switch element 60 comprises a control terminal 68 for changing therespective switch element 60 between two states, the first of whichcouples the first input terminal 62 to the output terminal 66 to passthe nominal power signal through the switch element 60 to the respectiveoutput connector 21 of the power multiplexor 16, the second of whichcouples the second input terminal 64 to the output terminal 66 to passthe attenuated power signal through the respective switch element 60 tothe respective output connector 21 of the power multiplexor 16.

The power multiplexor 16 further comprises a control circuit 70 coupledto the control terminals 68 of the respective switch elements 60 forcontrolling the timing of the switch elements 60. As previously stated,at any given time, the nominal power signal will be delivered to onlyone of the ablation probes 12, while the attenuating power signal willbe delivered to the remaining ablation probes 12 in accordance with theswitching pattern illustrated in FIG. 4. To this end, the controlcircuit 70 sequentially configures the switch device 59 between threedifferent states by configuring each switch element 60 to pass thenominal power signal to the respective output connector 21, whileconfiguring the remaining switch elements 60 to pass the attenuatedpower signal to the remaining respective output connectors 21.

That is, the control circuit 70 configures the switch device 59 in afirst state by configuring the first switch element 60(1) to pass thenominal power signal to the first output connector 21(1), whileconfiguring the second and third switch elements 60(2), (3) to pass theattenuated power signal to the second and third output connectors 21(2),(3). The control circuit 70 configures the switch device 59 in a secondstate by configuring the second switch element 60(2) to pass the nominalpower signal to the second output connector 21(2), while configuring thefirst and third switch elements 60(1), (3) to pass the attenuated powersignal to the first and third output connectors 21(1), (3). The controlcircuit 70 configures the switch device 59 in a third state byconfiguring the third switch element 60(3) to pass the nominal powersignal to the third output connector 21(3), while configuring the firstand second switch elements 60(1), (2) to pass the attenuated powersignal to the first and second output connectors 21(1), (2).

It should be noted that sequential configuration of the switch elements60 to pass the nominal power signal to the respective output connector21, and thus, the respective ablation probe 12, does not necessarilymean that the switch elements 60 are so configured in a numerical order,i.e., 1, 2, 3, etc. For instance, the first switch element 60 can beconfigured to pass the nominal power signal to the first outputconnector 21, then the third switch element 60, and then the secondswitch element 60.

The control circuit 70 may be configured to control the timing of theswitch device 59 in any one of a variety of manners. In the illustratedembodiment, the control circuit 70 is configured for switching the stateof the switch device 59 at a fixed frequency. Preferably, the fixedfrequency is at least once every second to maximize the efficiency ofthe tissue ablation system 10, but less than once every thirty secondsto prevent tissue charring otherwise resulting from providing nominalpower to an ablation probe for too long. The control circuit 70 may basethe switch timing on the operating frequency of the power signal. Forexample, if the operating frequency is 300 KHz, and it is desired tochange the state of the switch device 59 once a second, the controlcircuit 70 will change the state of the switch device 59 once every 300K cycles. Of course, rather than basing the switching frequency ondigital references, such as clock cycles, the switching frequency may bebased on analog references, such as capacitor discharge times.

Alternatively, the control circuit 70 may be configured to change thestate of the switch device 59 at a variable frequency. For example, thecontrol circuit 70 may change the state of the switch device 59 based onphysiological parameters, such as temperature or impedance, that can besensed and delivered back to the control circuit 70 during the ablationprocess (shown as dashed lines in FIG. 6). In the case of an impedancemeasurement, the control circuit can be connected to an electrode (notshown) on each ablation probe 12 and the dispersive electrode todetermine the impedance of the intervening tissue. In this case of atemperature measurement, a temperature sensor, such as a thermistor orthermocouple (not shown) can be mounted to an electrode of each ablationprobe 12 and then coupled to the control circuit 70.

In contrast to changing the state of the switch device 59 at a fixedfrequency, which may not optimal if the state of the switch device 59 ischanged to late or too early, the use of sensed physiological feedbackto the control circuit 70 allows the state of the switch device 59 to bechanged at the exact time that it needs to be changed. For example, asthe measured tissue impedance adjacent an ablation probe 12 that iscurrently delivered with the nominal power signal begins toexponentially increase or the measured temperature of the tissueadjacent the ablation probe 12 approaches 100° C., thereby indicatingthat full tissue ablation has been achieved, the control circuit 70 maychange the state of the switch device 59, so that the nominal powersignal is delivered to the next ablation probe 12, and attenuated powersignal is delivered to the previous ablation probe 12, along with theother remaining ablation probe 12. The control circuit 70 can thenchange the state of the switch device 59 based on the measured impedanceand/or temperature adjacent the next ablation probe 12 in the samemanner, and so on.

It should be noted that the use of physiological feedback parameters canbe used with other probe switching implantations besides those thatensure that power is continuously delivered to each ablation probe. Forexample, such a feature can be applied to prior art probe switchingimplementation, wherein power is delivered to one probe at a time, whileno power is delivered to the remaining probes.

Although the attenuation value of the attenuator 54 has been describedas being fixed, the power multiplexor 16 may be configured to vary theamplitude of the attenuated power signal delivered to the ablationprobes 12. In this case, the attenuator 54 has a variable attenuationvalue that is controlled by the control circuit 70 based on a userinput. For example, the power multiplexor 16 may have an attenuationcontrol device (not shown) that can be manipulated by the user based onthe characteristics of the tissue to be ablated, e.g., by inputting theattenuation value or by inputting the type of tissue to be ablated. Forexample, if the tissue is lung tissue, which requires a relatively smallamount of power to ablate, the user may input a relatively largeattenuation value or simply input the tissue type into the attenuationcontrol device. In contrast, if the tissue is liver tissue or otherwisetissue that is highly vascular, the user may input a relatively smallattenuation value or simply input the tissue type into the attenuationcontrol device. Whether the attenuation value is increased or decreased,the control circuit 70 will then respond to whichever type of input isused by varying the attenuation value of the attenuator 54.

Rather than basing the control of the attenuation value on a manualinput from the user, the attenuation value may be varied based on ameasured physiological parameter, such as tissue impedance ortemperature, in order to suit a longer probe dwell time, potentiallyresulting in a larger ablation. For example, if the measured tissueimpedance adjacent an ablation probe 12 to which the attenuated powersignal is currently delivered is relatively low or the measuredtemperature of such tissue is much less than 100° C., tissue ablationwill not likely occur in a relatively short time, and therefore, thecontrol circuit 70 may automatically decrease the attenuation value ofthe attenuator 54, so that the amplitude of the attenuated power signalis increased in order to quickly reach the point at which tissueablation occurs, thereby decreasing the elapsed time of the ablationprocedure. In contrast, as the measured tissue impedance adjacent anablation probe 12 to which the attenuated power signal is currentlydelivered begins to exponentially increase or the measured temperatureof the tissue adjacent the ablation probe 12 approaches 100° C., thecontrol circuit 70 may automatically increase the attenuation value ofthe attenuator 54, so that the amplitude of the attenuated power signalis decreased, thereby allowing the ablation probes 12 to have a longerdwell time, potentially creating a larger ablation.

The power multiplexor 16 can be implemented in any suitable manner thatfacilitates the afore-described attenuation and switching functions. Inthe illustrated embodiment, the attenuator 54 may be a discretecomponent that can be obtained from supplier, such as from JFWIndustries, Inc. The switch 59 and control circuit 70 can be implementedas a low power switching circuit to minimize the cost of the powermultiplexor 16. For example, the switch elements 60 take the form ofpowered transistors, and the control circuit can take the form of logiccircuitry. Alternatively, the switch elements 60 can take the form ofdiscrete components or electromechanical devices, such as relays.

Although the tissue treatment system 10 has been described as comprisingan equal number of ablation probes 12 and output connectors 21, a tissuetreatment system can optionally be configured, such that the number ofablation probes used can be less than the number of output portsavailable. For example, FIG. 7 illustrates a tissue treatment system 110that is similar to the previously described tissue treatment system 10,with the exception that it comprises a power multiplexor 116 with sixoutput connectors 21(1)-(6), although only three ablation probes 12 arestill used in this scenario.

The particular pattern followed by the power multiplexor 116 indelivering the power signals to the ablation probes 12 will depend onthe number of used output connectors 21; i.e., the number of outputconnectors 21 to which ablation probes 12 are connected. The powermultiplexor 116 will activate only those output connectors 21 that areused (in this case, output connectors 21(1)-(3)) and sequentially changeas many ablation states of the plurality of ablation probes 12 as thereare activated output connectors 21. The power multiplexor 116 will onlysequentially change three ablation states of the ablation probes 12,which will be accomplished in the same manner illustrated in FIG. 5, andwill deliver the nominal and attenuated power signals to the ablationprobes 12 in the same manner illustrated in FIG. 4.

If additional ablation probes 12 are used, thereby using additionaloutput connectors 21, the power multiplexor 166 will add additionalablation states between which the ablation probes 12 will change. Forexample, if three additional ablation probes 12 are added, thereby usingall six output connectors 21, the power multiplexor 166 willsequentially change six ablation states of the ablation probes 12 inaccordance with the switching pattern illustrated in FIG. 9, and willdeliver nominal and attenuated power signals to the six ablation probes12 connected to the used output connectors 21 in a manner similar tothat previously described, with the exception that nominal power signalwill be delivered to each of the six ablation probes 12, whiledelivering the attenuated power signal to the remaining five ablationprobes.

That is, as illustrated in FIG. 8, the power multiplexor 116 deliversthe nominal power signal to the first ablation probe 12(1) during afirst time period T1, while delivering the attenuated power signal tothe second to sixth ablation probes 12(2)-(6). The power multiplexor 116then delivers the nominal power signal to the second ablation probe12(2) during a second time period T2, while delivering the attenuatingpower signal to the first and third through fifth ablation probes 12(1),(3)-(6). This switching pattern continues for time periods T3-T6 andcontinuously repeats until the ablation procedure is completed.

The power multiplexor 116 may be implemented in the same manner as thepower multiplexor 16, with the exception that the power multiplexor 116comprises a switch 159 having six single pole three position (SP3P)switching elements 160. In particular, like the previously describedswitching elements 60, each of the switching elements 160 illustrated inFIG. 10 comprises a first input terminal 162 coupled to the first outputterminal 50 of the power splitter 46 for receiving the nominal powersignal, and a second input terminal 164 coupled to the output terminal58 of the power attenuator 54 for receiving the attenuated power signal.Unlike the previously described switching elements 60, each of theswitching elements 160 comprises a third input terminal 169 coupled toground.

Each switch element 160 also comprises a control terminal 168 forchanging the switch element 160 between three states, the first two ofwhich are similar to the states in which the previously described switchelements 60 can be placed. That is, each switch element 160 can beplaced in a first state that connects the first input terminal 162 tothe output terminal 166 to pass the nominal power signal through theswitch element 160 to the respective output connector 21, and a secondstate that connects the second input terminal 164 to the output terminal166 to pass the attenuated power signal through the switch element 160to the respective output connector 21. Unlike the previously describedswitch elements 60, each switch element 160 can be placed in a thirdstate that connects the grounded third input terminal 169 to the outputterminal 166 to prevent the passing of any power signal through therespective switch element 160 to the respective output connector 21. Asa result, the switch element 160 and the respective output connector 21are deactivated.

The power multiplexor 16 also comprises a control circuit 170 that issimilar to the previously described control circuit 70, with theexception that the control circuit 170 is configured to activate theused output connectors 21 by passing one of the nominal or attenuatedpower signals through the respective switch elements 160, and todeactivate the unused output connectors 21 by grounding the respectiveswitch elements 160. As discussed above, the control circuit 170 willsequentially change as many states of the switch device 159 as there areused output connectors 21.

In this case, the control circuit 170 will sequentially configure theswitch device 159 between three different states by configuring each ofthe first three switch elements 160(1)-(3) to pass the nominal powersignal to the respective one of the output connectors 21(1)-(3), whileconfiguring the remaining switch elements 160(1)-(3) to pass theattenuated power signal to the remaining respective output connectors21(1)-(3). The control circuit 170 will effectively accomplish thisfunction in the same manner that the control circuit 70 sequentiallyconfigured the switch device 59 described above. The main difference isthat the control circuit 170 will ground the switch elements 160(4)-(6)to prevent the passage of either the nominal power signal or theattenuated power signal to the respective output terminals 21(4)-(6). Ifadditional ablation probes 12 are added, thereby using additional outputconnectors 21, the control circuit 170 will configure the switch 159with additional states, allowing either of the nominal power signal orthe attenuated power signal to pass through the switch elements 160 tothe additional output connectors 21.

The control circuit 170 can determine the number of used outputconnectors 21 in any one of a variety of manners. For example, the powermultiplexor 16 can be provided with a user control (not shown) thatallows the user to manually set the number of ablation probes 12, andthus, output connectors 21, to be used. In this case, the user will haveto mate the ablation probes 12 to the output connectors 21 in apredetermined pattern. For example, if n number of ablation probes 12are to be used, the user will have to mate the ablation probes with thefirst n output connectors 21 of the power multiplexor 16. The controlcircuit 70 is configured to respond to the user entry of number ofablation probes 12 by activating the first n output connectors 21.

Alternatively, the control circuit 170 can be configured toautomatically sense the output connectors 21 that are to be used whenthe ablation probes 12 are mated with these output connectors 21. Forexample, mating of an ablation probe 12 to an output connector 21 mayinitiate a closed circuit condition that can be sensed by the controlcircuit 170. Accordingly, the control circuit 170 can then be configuredto activate the output connector 21 by grounding it through the switchelement 160 when the closed circuit condition is sensed. In contrast,removal of an ablation probe 12 from an output connector 21 may initiatean open circuit condition that can be sensed by the control circuit 170.Accordingly, the control circuit 170 can then be configured todeactivate the output connector 21 by grounding it through the switchelement 160 when the open circuit condition is sensed. Notably, in thiscase, the user need not mate the ablation probes 12 to the outputconnectors 21 in a predetermined pattern, since the control circuit 70can automatically sense the output connectors 21 that are to be used.

It should be noted that the adaptable probe activation feature can beused with other probe switching implantations besides those that ensurethat power is continuously delivered to each ablation probe. Forexample, such a feature can be applied to prior art probe switchingimplementation, wherein power is delivered to one probe at a time, whileno power is delivered to the remaining probes.

It should also be noted that although the previously described tissueablation systems 10, 110 have been described as being mono-polarablation systems, the attenuation-switching functions described abovecan be applied to bipolar ablation systems. In these types of systems,RF current is delivered to the electrode tines in a bipolar fashion,which means that current will pass between two electrode tines(“positive” and “negative” electrodes) of a single ablation probe, orbetween the electrode tines of the two different ablation probes. In theformer case, the tissue ablation system will generally perform theattenuation and switching functions in the same manner described above.In the latter case, some of the output connectors will operate aspositive poles to which nominal or attenuated power signals arepresented and delivered to the connected ablation probes, and some ofthe output connectors on the power multiplexor will be act as negativepoles from which nominal or attenuated power signals are received fromthe connected ablation probes after passage through intervening tissue.

Although the previous tissue ablation systems 10, 110 have beendescribed as providing power to the ablation probes using a single RFsource, it should be noted that multiple RF sources can be used toprovide the same attenuation and switching functions as a single RFsource—although such multiple RF sources may require more complex timingcircuitry in order to coordinate the different functions of the RFsources.

For example, one RF generator can be used to sequentially deliverednominal power signal to ablation probes, while delivering no powersignal to the remaining ablation probes to which the nominal powersignal is currently not being delivered. In this case, a powermultiplexor with no attenuation feature can be used to switch powerbetween the ablation probes. Another RF generator can be used to deliverattenuated power signal to the ablation probes that are currently notbeing delivered power from the first RF generator. Control circuitry ispreferably used to coordinate the timing of both RF generators, so thatwhen the nominal power signal is delivered to one ablation probe, theattenuated power signal is delivered to the other ablation probes.

As another example, a number of RF generators equal to the number ofablation probes can be used. In this case, each RF generator isdedicated to an ablation probe, and alternately provides nominal andattenuated power signals to that ablation probe. Control circuitry isused to maintain a phase difference between the RF generators, so thatthe nominal power signal is sequentially delivered to each ablationprobe, while the attenuated power signal is delivered to the remainingablation probes to which the nominal power signal is currently not beingdelivered.

Having described the structure of the tissue ablation system 10, itsoperation in treating targeted tissue will now be described. Thetreatment region may be located anywhere in the body where hyperthermicexposure may be beneficial. Most commonly, the treatment region willcomprise a solid tumor within an organ of the body, such as the liver,kidney, pancreas, breast, prostrate (not accessed via the urethra), andthe like. The volume to be treated will depend on the size of the tumoror other lesion, typically having a total volume from 1 cm³ to 150 cm³,and often from 2 cm³ to 35 cm³. However, the use of multiple ablationprobes lends itself to either the treatment of relatively large tumorsor a multiplicity of smaller tumors distributed within the patient'sbody. The peripheral dimensions of the treatment region may be regular,e.g., spherical or ellipsoidal, but will more usually be irregular. Thetreatment region may be identified using conventional imaging techniquescapable of elucidating a target tissue, e.g., tumor tissue, such asultrasonic scanning, magnetic resonance imaging (MRI), computer-assistedtomography (CAT), fluoroscopy, nuclear scanning (using radiolabeledtumor-specific probes), and the like. Preferred is the use of highresolution ultrasound of the tumor or other lesion being treated, eitherintraoperatively or externally.

Referring now to FIGS. 11A-11F, the operation of the tissue ablationsystem 10 is described in treating a treatment region TR within tissue Tlocated beneath the skin or an organ surface S of a patient. Although asingle treatment region TR is illustrated for purposes of brevity, thetissue ablation system 10 may alternatively be used to treat multipletreatment regions TR. The ablation probes 12 are first introducedthrough the tissue T, so that the distal ends 22 of the cannulae 18 arelocated respective target sites TS within the treatment region TR (FIG.11A).

This can be accomplished using any one of a variety of techniques. Inthe preferred method, each ablation probe 12 is percutaneouslyintroduced to the treatment region TR directly through the patient'sskin or through an open surgical incision. In this case, the distal endsof the cannulae 18 may be sharpened to facilitates introduction of theablation probes 12 to the treatment region TR. In such cases, it isdesirable that each cannula 18 be sufficiently rigid, i.e., have asufficient column strength, so that it can be accurately advancedthrough tissue T. In other cases, each cannula 18 may be introducedusing an internal stylet that is subsequently exchanged for the probeshaft 26. In this latter case, the probe shaft 26 can be relativelyflexible, since the initial column strength will be provided by thestylet. More alternatively, a component or element may be provided forintroducing each cannula 18 to the respective target ablation site TS.For example, a conventional sheath and sharpened obturator (stylet)assembly can be used to initially access the tissue T. The assembly canbe positioned under ultrasonic or other conventional imaging, with theobturator/stylet then removed to leave an access lumen through thesheath. The cannula 18 can then be introduced through the sheath lumen,so that the distal end 22 of the cannula 18 advances from the sheathinto the target ablation site TS.

Once the ablation probes 12 are properly positioned, the handle member36 of each ablation probe 12 is distally advanced to deploy theelectrode array 32 radially outward from the distal end 22 of therespective cannula 18 until the electrode array 32 fully everts withinthe respective target tissue site TS (FIG. 11B).

Once the electrode arrays 32 are fully deployed into the respectivetarget ablation sites TS, the RF generator 14 is connected to theablation probes 12 through the power multiplexor 16 (shown in FIG. 1).In accordance with the switching pattern illustrated in FIG. 4, the RFgenerator 14 and power multiplexor 16 are then operated to sequentiallydelivering nominal ablation energy (in this case, the nominal powersignal) to the ablation probes 12, while delivering the attenuatedablation energy (in this case, the attenuated power signal) to theablation probes 12 currently not delivered with the nominal ablationenergy. In other words, the ablation states of the ablation probes 12are sequentially changed in accordance with the states illustrated inFIG. 5.

In particular, the power multiplexor 16 places the ablation probes 12into the first ablation state by delivering the nominal ablation energyto the first ablation probe 12(1) to convey a relatively great amount ofablation energy into the tissue adjacent the first ablation probe 12(1),and by delivering the attenuated ablation energy to the second and thirdablation probes 12(2), (3) to convey a relatively small amount ofablation energy into the tissue adjacent the second and third ablationprobes 12(2), (3) (FIG. 11C). After a fixed period of time, oralternatively, after a measured physiological parameter indicates achange is necessary, the power multiplexor 16 places the ablation probes12 into the second ablation state by delivering the nominal ablationenergy to the second ablation probe 12(2) to convey a relatively greatamount of ablation energy into the tissue adjacent the second ablationprobe 12(2), and by delivering the attenuated ablation energy to thefirst and third ablation probes 12(1), (3) to convey a relatively smallamount of ablation energy into the tissue adjacent the first and thirdablation probes 12(1), (3) (FIG. 11D). After a fixed period of time orafter a measured physiological parameter indicates a change isnecessary, the power multiplexor 16 places the ablation probes 12 intothe third ablation state by delivering the nominal ablation energy tothe third ablation probe 12(3) to convey a relatively great amount ofablation energy into the tissue adjacent the third ablation probe 12(3),and by delivering the attenuated ablation energy to the first and secondablation probes 12(1), (2) to convey a relatively small amount ofablation energy into the tissue adjacent the first and third ablationprobes 12(1), (2) (FIG. 11E). The steps in FIGS. 11C-11E are repeateduntil the treatment region TR is completely ablated (FIG. 11F).

In an optional method, if the tissue ablation system 10 has a variableattenuation feature, the attenuation value of the power attenuator 54within the power multiplexor 16 may be manually set by the user prior toinitiation of the ablation procedure, or automatically and dynamicallyset by the power multiplexor 16 during the ablation procedure.

Operation of the tissue ablation system 110 will be similar to that ofthe tissue ablation system 10, with the exception that the powermultiplexor 116 will adapt to the number of ablation probes 12 usedeither after a manual input by the user or by automatically sensing thenumber of used ablation probes 12.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

1. A tissue ablation system, comprising: an ablation source having acommon power output; a power splitter having a splitter input and firstand second splitter outputs, the splitter input coupled to the commonpower output; a power attenuator having an attenuator input and anattenuator output, wherein the attenuator input is coupled to the secondsplitter output; and a switch device having first and second switchinputs respectively coupled to the first splitter output and theattenuator output, and a plurality of switch outputs coupleable orcoupled to a respective plurality of tissue ablation probes, wherein theswitch device is configured for sequentially coupling the first switchinput to each one of the switch outputs, while coupling the secondswitch input to the switch outputs currently not coupled to the firstswitch input.
 2. The tissue ablation system of claim 1, wherein theablation source is a radio frequency (RF) ablation source.
 3. The tissueablation system of claim 1, wherein the first and second splitteroutputs are substantially unattenuated relative to the common poweroutput.
 4. The tissue ablation system of claim 1, wherein the powerattenuator is variable.
 5. The tissue ablation system of claim 4,further comprising a feedback control circuit configured for receiving afeedback input and varying the power attenuator based on the feedbackinput.
 6. The tissue ablation system of claim 1, wherein the powerattenuator is configured for attenuating power at a level equal to orgreater than 3 dB.
 7. The tissue ablation system of claim 1, wherein thepower attenuator is configured for attenuating power at a level within arange of 3-6 dB.
 8. The tissue ablation system of claim 1, wherein theplurality of switch outputs comprises at least three outputs.
 9. Thetissue ablation system of claim 1, wherein the switch device isconfigured for sequentially coupling the first splitter output to eachswitch output at a fixed rate greater than once per second.
 10. Thetissue ablation system of claim 1, wherein the switch device isconfigured for sequentially coupling the first switch input to eachswitch output at a fixed rate less than once per thirty seconds.
 11. Thetissue ablation system of claim 1, further comprising a control circuitconfigured for receiving a feedback input and causing the switch deviceto sequentially couple the first switch input to each switch outputbased on the feedback input.
 12. The tissue ablation system of claim 1,further comprising the tissue ablation probes.